Electropolishing of titanium base alloys



, amide.

United States Patent 3,087,874 ELECTROPOLISHING 0F TITANIUM BASE ALLOYSDon H. Greisl, Cincinnati, Ohio, and Robert Edward Thompson, Westaburg,and William Marion Spurgeon, South'iield, Mich., assignors to the UnitedStates of America as represented by the Secretary of the All Force NoDrawing. Filed Feb. 13, 1%1, Ser. No. 89,080 2 Claims. (Cl. 204-1405)This invention relates to an electropolishing solution and to a methodfor electropolishing titanium base alloys.

During the fabrication of structural elements composed of titanium basealloys, it often becomes necessary to polish the surfaces of suchelements and convert them to a high luster. In general, polishedsurfaces are obtained by buifing, mechanical polishing, electroplatingor electropolishing. Electropolishing has been found to be verydesirable because it is economical and provides a bright and highlylustrous surface on most metals and their alloys. Electropolishing, as aprocess, generally comprises the steps of immersing a metal object to bepolished, as an anode, in an electrolytic solution and passing a currentthrough the solution between the metal object being polished and anotherobject referred to as the cathode. The electrolytic solutions employedheretofore in the electropolishing of metals and their alloys areaqueous acid solutions generally containing perchloric or hydrofluoricacid as their principal constituent. However, such solutions are highlycorrosive in character and are potentially explosive. They are difficultto handle and oftentimes violent in action. In addition, the use ofthese solutions often produced semi-polished areas as well asundesirable pits in the surface areas. A further problem resides in thefact that titanium base alloys themselves are diificult to electropolishbecause of their inability to pass current as an anode in most aqueouselectrolytes.

In accordance with this invention, however, it has been found that thesurface of titanium base alloys can be successfully polished to a smoothbright lustre by employing an electrolytic solution which containsammonium bifiuoride, fluoboric acid, hydrochloric acid and form- Thissolution is relatively economical in use and produces a highly effectivesolvent action on the surface of titanium base alloys. The potentialexplosiveness and highly corrosive nature of prior art solutions areeliminated, thereby contributing to the ease with which the solutions ofthe present invention are handled.

Accordingly, it is an object of the present invention to provide amethod and a solution for electropolishing the surface areas of titaniumbase alloys.

Another object of the present invention is to provide a method andsolution for electropolishing titanium base alloys which obviates theuse of highly corrosive and potentially explosive acid solutions.

A further object of this invention is to provide a method and solutionfor electropolishing which is effective in producing pit-free areas ofhigh lustre on the surfaces of titanium base alloys.

The above and still further objects, advantages, and features of thepresent invention will become readily apparent upon consideration of thefollowing detailed disclosure thereof.

In practicing the process of this invention, an electrolytic solution isplaced in a suitable container and a titanium base alloy, to bepolished, is immersed therein. This solution, which will be described ingreater detail hereinafter, is heated to a temperature of from about 88F. to 96 F. The particular titanium alloy employed is composed of 7percent aluminum, 4 percent molybdenum, with the balance beingsubstantially all titanium.

3,087,874 Patented Apr. 30, 1963 "ice The titanium alloy which is to bepolished is connected to a copper wire or some other metal wiresufficiently resistant to attack by the electrolyte and is immersed inthe electrolytic bath to serve as the anode. Carbon, or some othermaterial suit-able for use as a cathode, is likewise connected to thecopper wire and immersed in the electrolyte.

A suitable source of electric power is connected to the anode andcathode by means of the copper Wire in order to complete the circuit. Anelectric current is passed through the circuit at an anodic currentdensity sufficient to produce a highly polished area on the surfaces ofthe titanium alloy anode. It has been found that current densities offrom about 3 to 5 amperes per square inch are highly successful andprovide a surface removal rate of about 0.001 inch per minute. Anammeter and a resistance rheostat may be included in the circuit as ameans of controlling the anodic current density of the electrolyticbath.

With the foregoing discussion in mind there is presented herewith adetailed specific example of the electrolytic solution of this inventionHowever, the invention is not intended to be limited thereby. Thecomposition was prepared by mixing together the following ingredientsper liter of solution.

The above solution, when heated to a temperature of from about 88 F. to96 F., successfully electropolishes the surface of a titanium base alloyat an anodic current density of from about 3 to 5 .amperes per squareinch in accordance with the method of this invention discussed in detailhereinbefore. The titanium alloy anode dissolves and loses metal fromits surface at the rate of about 0.001 inch per minute. The anodesurface is evenly attacked by the electrolytic solution of thisinvention .and provides a bright and lustrous surface thereon. Theelectropolish ing action extends to all surfaces of the alloy in auniform manner with no undesirable pitting. Fairly low voltages areemployed with the current densities discussed heretofore. The use ofhigher current densities result in shorter polishing times and may beemployed if desired. However, uneven polishing and pitting are likely toresult. The time required by this invention to effectuateelectropolishing of a titanium alloy is not critical. However, excellentresults have been obtained on the specific titanium alloy discussedheretofore when it was immersed in the electrolytic solution of thisinvention for a period of from about 5 to 10 minutes.

The present application has described a bath type process such as isemployed with metal sheets or plates. However, it is to be understoodthat various modifications could be employed for effectuating thecontinuous treatment of strip-like metals.

It is to be further understood that although this invention has beendescribed with reference to specific embodiments, various modificationsand alterations can be made by those skiled in the .art withoutdeparting from the spirit and scope of the invention, and that all suchmodifications as fall within the scope of the appended claims areintended to be included therein.

What is claimed is:

1. An electropolishing solution for treating the surfaces of a titaniumbase .alloy consisting essentially of 22 to 28 grams of ammoniumbifluoride, 14 ml of fluoboric acid, 441 ml. of hydrochloric acid, and532 ml. of formamide per liter of solution.

2. A process for electropolishing a titanium base alloy which comprisesthe steps of immersing said alloy in an electrolytic solution, saidelectrolytic solution consisting essentially of about 22 to 28 grams ofammonium bifluoride, about 14 ml. of fluoboric acid, about 441 ml. ofhydrochloric acid and about 532 ml. of formamide per liter of saidsolution; making said alloy the anode in an electrolytic circuit whichincludes said electrolytic solution, a cathode, and a suitable source ofelectric power; heating said electrolytic solution to an elevatedtemperature of from about 88 F. to 96 F.; and passing an electriccurrent through said circuit at a current density of about 3 to 5amperes per square inch for a time period of from about 5 to 10 minutes.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES ASTM Standards, 1942, Part I, pages 1493-1496. Jacquet: MetalTreatment and Drop Forging, April 1951, page 176.

2. A PROCESS FOR ELECTROPOLISHING A TITANIUM BASE ALLOY WHICH COMPRISESTHE STEPS OF IMMERSING SAID ALLOY IN AN ELECTROLYTIC SOLUTION, SAIDELECTROLYTIC SOLUTION CONSISTING ESSENTIALLY OF ABOUT 22 TO 28 GRAMS OFAMMONIUM BIFLUORIDE, ABOUT 14 ML. OF FLUOBORIC ACID, ABOUT 441 ML. OFHYDROCHLORIC ACID AND ABOUT 532 ML. OF FORMAMIDE PER LITER OF SAIDSOLUTION; MAKING SAID ALLOY THE ANODE IN AN ELECTROLYTIC CIRCUIT WHICHINCLUDES SAID ELECTROLYTIC SOLUTION, A CATHODE, AND A SUITABLE SOURCE OFELECTRIC POWER; HEATING SAID ELECTROLYTIC SOLUTION TO AN ELEVATEDTEMPERATURE OF FROM ABOUT 88*F. TO 96*F.; AND PASSING AN ELECTRICCURRENT THROUGH SAID CIRCUIT AT A CURRENT DENSITY OF ABOUT 3 TO 5AMPERES PER SQUARE INCH FOR A TIME PERIOD OF FROM ABOUT 5 TO 10 MINUTES.